1. Field of the Invention
The invention relates to semiconductor device fabrication and, more particularly, to a method and apparatus for imparting a coating material to the surface of a semiconductor wafer and for efficiently removing excess amounts of the coating material and preventing excess coating material from adhering to the edges and back side of the wafer once the wafer has been coated.
2. Description of the Related Art
As is well known to those experienced in the art of semiconductor device fabrication, polyimide films have proven useful for a number of purposes, particularly for planarization of the semiconductor device surface in anticipation of photolithographic patterning and a passivation stress buffer. Polyimide films, which consist of polyimide resins in a solvent such as NMP (n-methyl-2-pyrolidone), demonstrate excellent thermal stability and chemical resistance, as well as acceptable dielectric properties. Consequently, polyimide films can be left permanently in place on the device, and are often used as a substitute for SiO.sub.2 or PSG (phososilicate glass) layers. The films can be applied in layers on the order of 1 to 40 .mu.m thickness by conventional photoresist spinning techniques. Upon application, polyimide films flow around projections and steps in the device surface, thereby contributing to planarization. The film contour becomes permanent upon curing at about 300.degree. C. In use, the degree of planarization effected by polyimide coating may be controlled by the viscosity of the film, that is, by the ratio of resin to solvent, and by the film thickness.
In a conventional semiconductor device fabrication process, a polyimide coating step takes place in a coater spin cup. The semiconductor wafer is supported inside the cup by a spin chuck. A dollop of polyimide is dispensed on the top surface of the wafer, and the wafer is rotated through rotation of the spin chuck shaft, while the coater spin cup remains essentially stationary. As the wafer rotates, polyimide is more or less uniformly dispensed over the surface of the wafer, with excess polyimide material centrifugally expelled off the circumferential perimeter of the wafer. The wafer then is subjected to a backrinse with a solvent such as propylene glycol monomethyl ether acetate (PGMEA). As an undesired side effect of the coating process, excess polyimide material is deposited on the edges of the wafer and accumulates in the coater spin cup, thereby requiring cups to be periodically changed out after, approximately, 100 to 150 coating steps. In addition, because polyimide mixed with PGMEA results in a viscous fluid that cannot be piped into a bulk drain, the polyimide must be evacuated to a waste drain tank.
Accordingly, what is desired is an improved polyimide coating step in a semiconductor device fabrication process, wherein the improved step mitigates the need for frequent changes in the coater spin cup, removes wafer-edge and back side deposits and provides more efficient and convenient disposal of excess polyimide.